The present invention relates to seals in a gas turbine for supplementing the chordal hinge seals between turbine nozzles and a turbine nozzle support ring and particularly relates to supplementary seals for substantially minimizing or eliminating leakage losses past the chordal hinge seals.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer bands. A nozzle retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner band and supports the first-stage nozzles against axial movement.
In an exemplary arrangement, eighteen cast segments are provided with two vanes per segment. The annular array of segments are sealed one to the other along adjoining circumferential edges by side seals. The side seals seal between a high pressure region radially inwardly of the inner band, i.e., compressor discharge air at high pressure, and the hot gases of combustion in the hot gas flow path which are at a lower pressure.
Chordal hinge seals are used to seal between the inner band of the first-stage nozzles and an axially facing surface of the nozzle support ring. Each chordal hinge seal includes an axial projection which extends linearly along a chord line of the inner band portion of each nozzle segment. Particularly, the chordal hinge seal extends along an inner rail of each segment and which rail extends radially inwardly of the inner band portion. The chordal hinge seal projection lies in sealing engagement with the axially opposite facing sealing surface of the nozzle support ring.
During operation and/or repair of the first-stage nozzle, it has been found that warpage can leave gaps between the chordal hinge seals and the sealing surface of the nozzle support ring. These gaps enable leakage past the chordal hinge seals from the high pressure area radially within the annular inner band into the hot gas flow path. That is, the chordal hinge seals are inadequate to prevent leakage flow as the chordal hinge seal projections lose contact with the sealing surface of the nozzle support ring. Consequently, there is a need for a supplemental seal at the interface of the first-stage nozzles and nozzle support ring to minimize or eliminate the leakage flow past the chordal hinge seals.
In accordance with a preferred embodiment of the present invention, there is provided a supplemental seal between the first-stage nozzles and the nozzle support ring which eliminates or minimizes leakage past, the chordal hinge seals. The supplemental seal includes flexible, preferably sheet metal, seals secured to one of the first and second axially facing surfaces of the nozzle support ring and nozzle segments, respectively, and preferably to the sealing surface of the nozzle segments. Each flexible seal extends arcuately along the nozzle segments radially outwardly of the primary seal formed by the chordal hinge seal. In an exemplary embodiment, the sealing surfaces of the nozzle segments are provided with a circumferentially extending groove which receives a margin of the flexible seal. Particularly, the margin of the flexible seal received in the groove is bent or folded to bear in sealing engagement against a surface, preferably the base of the groove. To retain the margin of the flexible sheet metal seal in the groove, an elongated strip is secured to the flexible seal along its margin thereof, the flexible seal being slidable circumferentially into the groove upon installation. The groove has a flange and the strip includes an opposing overlapping flange which retains the flexible seal against axial dislocation from the groove. Preferably a second elongated strip is provided along the opposite side of the margin from the first strip to locate the edge of the strip centrally of the groove.
The opposite margin of the flexible seal includes an edge which bears against the annular first sealing surface of the nozzle support ring. Because the flexible seal is located radially outwardly of the chordal seal, leakage flow past the chordal seal bears against the radial inner surface of the flexible sheet metal seal to maintain the edge of the flexible seal in sealing engagement with the first sealing surface of the nozzle support ring. While the flexible seal can be made in circumferential segments corresponding to the extent of each nozzle segment, preferably the flexible seal is provided in two or four circumferentially extending segments. In this manner, the flexible seal spans the joint between adjacent segments which enhances the sealing capacity of the supplemental seal.
In a preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a turbine nozzle segment having at least one stator vane and including an inner band, the segment having a second surface in axial opposition to the first surface, one of the first and second surfaces including an axially extending projection therealong for engagement with another of the first and second surfaces to form a first seal therebetween sealing between high and low pressure regions on opposite sides of the seal and a flexible seal extending between the first and second surfaces and between opposite sides of the segment, a first margin of the flexible seal being secured to one of the first and second surfaces and a second margin of the flexible seal sealingly engageable with another of the first and second surfaces to seal against another surface.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a plurality of turbine nozzle segments each having at least one stator vane and forming a generally annular second surface in axial opposition to the first surface, each of the segments including an axially extending projection along the second surface thereof for engagement with the first surface to form a first seal therebetween sealing between high and low pressure regions on opposite sides of the seal, a plurality of flexible seal segments extending between the first and second surfaces, each flexible seal segment having a first margin secured to a portion of the second surface and spanning at least one joint between adjacent segments, a second margin of each flexible seal segment being sealingly engageable with the first surface in response to leakage flow past the first seal.